A webhook is how a payment result reaches your server rather than only your browser. You make one with a URL, method, headers, and retry rules, then attach it to a gateway.
These pages cover the mechanics inside MockGateway. For background on what payment webhooks are and why signature checks matter, the blog post <a href="/blog/how-payment-gateway-webhooks-work">How Payment Gateway Webhooks Work</a> covers the concept and is not repeated here.